Automatic gain control circuit



J' FAMPL Sept. 6, 1938. W. VAN B. ROBERTS 2,129,028

AUTOMATIC GAIN CONTROL CIRCUIT v K Filed June 23, 1936 2 Sheets-SheetHIII lr-f l E AMPL vvvvv vv [.EAMPL.

vvvvv n WALTER VAN B. ROBERTS mm BY l I ATTORNEY Sept. 6, 1938. w. VANB.ROBERTS AUTOMATIC GIN CONTROL CIRCUIT Filed June 25, 1936 2 Sheets-Sheet2 SIGNAL STRENGTH SIGNAL STRENGTH SIGNA L STRENGTH SIGNAL STRENGTHINVENTOR WALTER \IAN B. ROBERTS ATTORNEY Patented Sept. 6, 1938 UNITEDSTATES PATENT OFFICE AUTOMATIC GAIN CONTROL CIRCUIT of DelawareApplication June 23, 1936, Serial No. 86,722

1 Claim.

My present invention relates to gain control circuits for radioreceivers, and more particularly to gain control arrangements for radioreceivers f 15 be arranged to control the signal transmission' tubespreceding the point in the signal transmission channel from which thesignal energy for control is taken, and the gain control may also beapplied to transmission tubes following the 20 point from which signalenergy for control is derived. For example, such gain controlarrangements have been disclosed by G. L. Beers in U. S. P. 1,960,723 ofMay 29, 1934.

It may be stated that it is one of the main 25 objects of my presentinvention to apply gain control to a signal amplifier preceding thereceiver demodulator, and which amplifier follows the point in thesignal transmission channel from which signal energy for controlpurposes 30 is derived, the signal input to the controlled amplifierbeing maintained substantially uniform by the automatic gain controlarrangement, and there being provided a device for adjusting thesensitivity of the controlled amplin 35 er feeding the demodulatorindependently of an auxiliary means which functions to vary the gaincontrol applied to the last named amplifier, whereby different relationsbetween received signa1 strength and receiver output may be secured. 40Another important object of the invention may be stated to reside in theprovision, in a radio receiver, of devices which may be adjusted tocontrol the gain of an ampliiier feeding into the receiver demodulator,such devices being adjust- 45 able to vary at will the receiver outputfor different conditions of received signal amplitude; and the receiveroutput adjustment devices essentially comprising a manually adjustableinstrumentality for varying the gain of the ampli- 50 iier feeding intothe demodulator, and an automatic gain control arrangement which isconstructed to apply different degrees of gain control tothe amplifier,the amplifier additionally having the signal amplitude at its inputcircuit 55 maintained substantially uniform.

Another object of the invention may be stated to reside in the provisionof a superheterodyne receiver which includes an IF amplifier feedinginto the second detector, an automatic volume control circuit beingprovided to control at least 5f one of the stages preceding the IFamplifier in a sense such that the signal amplitude at the IF amplifierinput circuit is maintained substantially uniform over a relatively widerange of sigy nal amplitude variation at the receiver signal collector,and the control circuit, furthermore, being constructed so as to receivesignal energy for control purposes from a point ahead of the IFampliiier to applying gain control to the IF amplifier tube, the IFamplifier including a manually adjustable device for controlling in apredetermined manner the amplitude of weak signais prior to demodulationby the second detector, and the automatic volume control circuitincluding a device for varying the degree of gain control on the IFamplier in such a manner that the transmission of strong signals to thesecond detector is effectively predetermined.

Still other objects of the invention are to improve generally theefficiency of receivers employing automatic gain control arrangements,and more especially to provide a superheterodyne receiver adapted to bevaried from local to distant reception in a simple and effective manner,and which receiver is moreover efficient in operation, and economicallymanufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claim; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried intoeffect.

In the drawings,

Fig. 1 schematically shows a superheterodyne receiver embodying theinvention,

Fig. 2 shows four receiver output-signal strength characteristics of thereceiving system shown in Fig. l for diiferent adjustments of the deviceembodied in the invention.

Referring now to Fig. l, there is shown in schematic manner asuperheterodyne receiver of a conventional type. The receiver maycomprise the usual signal collector A, and this may be of the customarygrounded antenna type, or a radio frequency distribution line, or' eventhe signal 55V collector employed on mobile structures such asautomobiles. Regardless of the construction of the collector, thecollected signals, which may be in the broadcast range of 500 to 1500kc., are impressed on the tunable input circuit of the radio frequencyamplier tube 2. The radio frequency amplifier is constructed in theusual manner and includes the signal'grid biasing network 3 in itscathode circuit, the function of the network 3 being to provide theminimum signal grid bias,

and the amplifier tube having its maximum gain at that normal biasvalue. The ampliiied signals are fed to the tunable input circuit 4 ofthe converter network 5. The latter network may comprise independentfirst detector and local oscillator tubes constructed in any well knownmanner, or it may comprise a tube, such as one of the 2A7 type, whereinthe electrodes are connected by circuits to provide a combined localoscillator'- Iirst detector stage. The numeral S denotes the tunablelocal oscillator circuit, whatever the construction of network 5, andthe numeral 'I signifies the IF output circuit of the converter network5. The variable condensers of circuits I, 4 and 6 are arranged to havetheir rotors mechanically controlled in uni-control fashion, and thedotted line 8 signies such uni-control mechanism. As those skilled inthe art will readily understand, the tuning of the circuit 5 is Variedover a frequency range which differs from the frequency range ofcircuits I and 4 by an intermediate `frequency to which the circuit I isresonated. This IF value may be chosen from` a range of 75 to 465 kc.,and where the receiver is of the multi-range type it will be understoodthat the operating IF may assume different values depending on thedesign of the transmission circuits. Furthermore, it will be understoodthat the customary devices may be used in the oscillator tank circuit 5so as to maintain the energy in circuit 'I at a substantially constantIF value regardless of what frequency the tunable circuits are adjustedto.

The IF energy in circuit 'I is transmitted through the IF ampliernetwork, and the latter is shown as comprising at least two ampliers 9and IB. The amplier 9 has its IF tuned circuit coupled to the circuit"I, and the output circuit I2 on ampliier 9 is coupled to the inputcircuit I3 of amplifier III. Circuits I2 and I3 are resonated to theoperating IF value. The numeral I4 designates the IF transformer, havingprimary and secondary circuits each tuned to the operating IF, whichtransmits the amplied IF energy to the demodulator, or second detector.The demodulator, and its following audio amplier network and reproducer,is not shown because those skilled in the art are fully aware of theconstructional details of such stages. The IF amplifier 9 includes inits cathode circuit the signal grid biasing network I5, and the amplierIl] includes in its connection to ground a resistor I5 which isadjustable in magnitude. 'Ihe IF by-pass condenser I'I is connectedacross resistor I6, a source of direct current voltage (not shown) beingconnected in shunt across adjusttable resistor IS, and the cathode sideof resistor I6 being connected through the resistor I8 to the positiveterminal of the voltage' source shunting resistor I6. It will then beseen that by adjusting the magnitude of resistor I6 the signal inputgrid of tube I I! may be given a predetermined negative bias withrespect to the cathode. The adjustable resistor I6 functions as themanual gain contro-l mechanism or device for IF amplier I0, and itsfunctional relation to the automatic gain control circuit will bedescribed at a later point.

The automatic gain control arrangement comprises a diode IS havingconnected in series between its anode and grounded cathode a pathincluding the resonant input circuit 2|) and the load resistor 2|. TheIF energy is impressed on input circuit 2t through an amplication stage,and for this purpose the amplier tube'2I has its resonant output circuit22 coupled to input 'circuit 20. Circuits 20 and 22 are tuned to theoperating IF, and the signal grid of tube 2| is connected to the highalternating potential side of circuit |I through a coupling condenser22. The amplifier tube 2| includes the usual biasing network 23 in itsgrounded cathode lead, and the signal grid of tube 2l is connected toground through arresistor 24 so that a desired operating bias may beapplied to the signal grid of tube 2l. 'Ihe direct current Voltagedeveloped across resistor 2| by the rectified IF currents, is impressedupon the signal grid circuits of diierent signal transmission tubes ofthe receiver. Thus, the anode side of resistor 2| isi connected to thelow alternating potential side of input I through a path which includesthe lead 25, and this lead is designated by the letters AVC to show thatthis is the automatic volume control connection for the radio frequencyamplifier tube.

The AVC lead 25 may include appropriate lter elements 3D and 3|, and thefunction of these lter elements is to suppress the pulsating voltagecomponents developed across resistor 2|. The signal grid of the firstdetector may also be connected to the AVC lead 25 if desired. The firstIF amplifier 9 has its signal grid circuit connected to the AVC rectierI9 so that less AVC voltage is impressed on amplifier 9 than is appliedto amplifier 2. This is accomplished by connecting the low alternatingpotential side of input circuit II to a voltage reducing resistor 32through the AVC lead 33. The resistor 32 is connected from lead 25 toground, and the filter elements 34 are inserted in the AVC connection 33to suppress pulsating voltages.

The signal grid of amplier IB is connected to a desired point onresistor 2|' through an adjustable tap element 4I). The tap 40 may beadjusted to any point along resistor 2|', and it is connected throughthe AVC lead 4I to the low alternating potential side of input circuitI3. The AVC lead 4| includes lter elements for suppressing pulsatingvoltages. It will therefore be seen that the automatic volume controlnetwork operates its signal energy for control pur- Y poses from theinput circuit of IF amplier 9,

but it not only controls the gain of amplier 2 which precedes inputcircuit II, but also controls the gain of each of amplifiers 9 and I0which follow the input circuit I i. Furthermore, while the grid circuitsof amplifiers 2 and 9 are always connected to a common point on resistor2|', the AVC connection 4| to amplier IB may be made to any desiredpoint of resistor 2|', and for a purpose now to be described in detail.

Considering the amplier I0 it will be seen that the gain control actionof amplifiers 2 and 9 results in the maintenance of the signal amplitudeat input circuit I3 substantially uniform over a wide variation ofsignal amplitude at collector A. The action that secures this result iswell known and it is only necessary to state that the signal energy frominput circuit is amplied by amplifier 2|, and then rectified by creases.

diode I9. The signal grids of ampliers 2 and 9 being connected to thepoint on resistor 2|', which is negative with respect to ground, thegain of each of amplifiers 2 and 9 will be decreased as the receivedsignal amplitude in- Assume, now, that the tap 40 is adjusted to thepoint on resistor 2| to which lead 25 is connected. Further, let it beassumed that the resistor I6 has been adjusted so that in the absence ofreceived signals the amplier I has a normal negative bias impressedthereon such as to give maximum gain. In such case the characteristic ain Fig. 2 shows the receiver output-signal strength characteristic ofthe receiving system. It will be seen that with these adjustments of tap40 and variable resistor I6 weak stations will only be heard Whereas theresponse to stronger stations Will be practically eliminated by virtueof the connection of AVC lead lII to the most negative point on resistor2|', It will therefore be seen that by adjusting the manual sensitivitycontrol I6 for maximum sensitivity, and adjusting tap 40 for maximumgain control action on amplifier I0, response is limited practically toWeak stations, as shown by characteristic a in Fig. 2.

If now tap 4U is shifted to the grounded side of resistor 2 I', and themanual sensitivity control I6 is adjusted to reduce the sensitivity ofamplier I0 from its previous maximum setting, then characteristic b issecured. From this characteristic it will be seen that the weak signalresponse is a minimum, whereas the strong signal response is a maximum.With these adjustments of tap 4I] and manual sensitivity control I6, theweak signal response is practically eliminated. The automatic gaincontrol action on amplifier I0 in this case is also eliminated, and gainregulation depends upon the regulation of amplifiers 2 and 9.

The characteristic c is secured by restoring maximum sensitivity controlI6 to the maximum sensitivity setting, but with the tap 40 still at thegrounded side of resistor 2|'. In this case it is seen thatthe responseto Weak signals is increased, and this results because of the increaseof the normal operating gain of amplifier IU. Here, again, the gainregulation depends upon the control applied to amplifiers 2 and 9.

The characteristic d is secured by adjusting the tap 40 to anintermediate point on resistor 2|', and adjusting the sensitivitycontrol I6 to a moderate setting. The characteristic secured by theseadjustments is such that the receiver response is confined to anintermediate range of signal amplitude values. It will be seen that withcharacteristic d the response to weak signals and strong signals ispractically eliminated. 'I'his characteristic results because of thefact that the control device I6 has been adjusted so as to reduce thenormal sensitivity of amplier Il) to a value below maximum, and alsobecause of the fact that the gain control applied to amplifier I0 is ofan intermediate degree.

It will now be seen that by proper adjustment of the manual sensitivitycontrol I6 and the automatic gain control tap 4D the different receiveroutput-signal strength characteristics sho-wn in Fig. 2 may be secured.In other words, the receiver response to Weak signals, signals ofmoderate amplitude, or strong signals may be varied at the will of thereceiver operator. Clearly, for distant reception the characteristic ais desirable, Whereas for local reception the characteristic b isdesired. By means of the present invention the receiver output withrespect to rec'eived signal amplitude becomes adjustable so as tosatisfy varying reception conditions.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modiications may be made without departingfrom the scope of my invention, as set forth in the appended claim.

What is claimed is:

In a receiving system provided withapluralityof signal transmissiontubes arranged in a cascade network, manually adjustable meanselectrically associated with the last of said tubes for establishing thetube gain at a predetermined value related to a desired signalamplitude, automatic volume control means having a signal input circuitconnected to an intermediate point in said cascaded tube network, animpedance electrically associated with said control means for developingthereacross a direct current voltage which varies in accordance withsignal amplitude variation, a direct current voltage connection betweena point on said impedance and a gain control electrode of at least oneof the tubes preceding said intermediate point, a second direct currentvoltage connection between a gain control electrode of said last tubeand a point on said impedance, and means for selecting at will the pointof connection to the impedance of said second direct current voltageconnection whereby there may be adjusted the rate of reduction of gainof said last tube with increase in signal amplitude, said manuallyadjustable means and said selecting means having gain control adjustingranges such that the receiver response to signals of differentamplitudes may be varied widely at the will of the receiver operator.

WALTER VAN B. ROBERTS.

